CSS Transitions

Abstract

CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.

Status of this document

The list of changes made to this specification is available.

Table of contents

Introduction

This section is not normative.

This document introduces new CSS features to enable implicit transitions, which describe how CSS properties can be made to change smoothly from one value to another over a given duration.

Transitions

Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.

For example, suppose that transitions of one second have been defined on the 'left' and 'background-color' properties. The following diagram illustrates the effect of updating those properties on an element, in this case moving it to the right and changing the background from red to blue. This assumes other transition parameters still have their default values.

Transitions of 'left' and 'background-color'

Transitions are a presentational effect. The computed value of a property transitions over time from the old value to the new value. Therefore if a script queries the computed style of a property as it is transitioning, it will see an intermediate value that represents the current animated value of the property.

Only animatable CSS properties can be transitioned. See the table at the end of this document for a list of properties that are animatable.

The transition for a property is defined using a number of new properties. For example:

  div {
    transition-property: opacity;
    transition-duration: 2s;
  }
  

Each of the transition properties accepts a comma-separated list, allowing multiple transitions to be defined, each acting on a different property. In this case, the individual transitions take their parameters from the same index in all the lists. For example:

  div {
    transition-property: opacity, left;
    transition-duration: 2s, 4s;
  }

  

In the case where the list of values in transition properties do not have the same length, the list is repeated as a whole in order to provide necessary values.

Issue: Are the lists repeated to all be the length of the longest list, or are they repeated or truncated to match the length of the 'transition-property' list?

      div {
        transition-property: opacity, left, top, width;
        transition-duration: 2s, 1s;
      }
      

The 'transition-property' Property

The 'transition-property' property specifies the name of the CSS property to which the transition is applied.

A value of 'none' means that no property will transition. A value of 'all' means that every property that is able to undergo a transition will do so. Otherwise, a list of properties to be transitioned is given.

If one of the identifiers listed is not a recognized property name or is not an animatable property, the implementation must still start transitions on the animatable properties in the list using the duration, delay, and timing function at their respective indices in the lists for 'transition-duration', 'transition-delay', and 'transition-timing-function'. In other words, unrecognized or non-animatable properties must be kept in the list to preserve the matching of indices.

Are 'all', 'none', 'inherit', and 'initial' allowed as items in a list of identifiers (of length greater than one)?

If one of the identifiers listed is a shorthand property, implementations must start transitions for any of its longhand sub-properties that are animatable, using the duration, delay, and timing function at the index corresponding to the shorthand.

If a property is specified multiple times in the value of 'transition-property' (either on its own or via a shorthand that contains it), then the transition that starts uses the duration, delay, and timing function at the index corresponding to the last occurrence of the property.

The 'transition-duration' Property

The 'transition-duration' property defines the length of time that a transition takes.

This property specifies how long the transition from the old value to the new value should take. By default the value is '0', meaning that the transition is immediate (i.e. there will be no animation). A negative value for transition-duration is treated as '0'.

The 'transition-timing-function' Property

The 'transition-timing-function' property describes how the intermediate values used during a transition will be calculated. It allows for a transition to change speed over its duration. These effects are commonly called easing functions. In either case, a mathematical function that provides a smooth curve is used.

Timing functions are either defined as a stepping function or a cubic bezier curve. The timing function takes as its input the current elapsed percentage of the transition duration and outputs a percentage that determines how close the transition is to its goal state.

A stepping function is defined by a number that divides the domain of operation into equally sized intervals. Each subsequent interval is a equal step closer to the goal state. The function also specifies whether the change in output percentage happens at the start or end of the interval (in other words, if 0% on the input percentage is the point of initial change).

Step timing functions

A cubic bezier curve is defined by four control points, P₀ through P₃ (see Figure 1). P₀ and P₃ are always set to (0,0) and (1,1). The 'transition-timing-function' property is used to specify the values for points P₁ and P₂. These can be set to preset values using the keywords listed below, or can be set to specific values using the 'cubic-bezier' function. In the 'cubic-bezier' function, P₁ and P₂ are each specified by both an X and Y value.

Bezier Timing Function Control Points

The timing functions have the following definitions.

ease

The ease function is equivalent to cubic-bezier(0.25, 0.1, 0.25, 1.0).

linear

The linear function is equivalent to cubic-bezier(0.0, 0.0, 1.0, 1.0).

ease-in

The ease-in function is equivalent to cubic-bezier(0.42, 0, 1.0, 1.0).

ease-out

The ease-out function is equivalent to cubic-bezier(0, 0, 0.58, 1.0).

ease-in-out

The ease-in-out function is equivalent to cubic-bezier(0.42, 0, 0.58, 1.0)

step-start

The step-start function is equivalent to steps(1, start).

step-end

The step-end function is equivalent to steps(1, end).

steps

Specifies a stepping function, described above, taking two parameters. The first parameter specifies the number of intervals in the function. It must be a positive integer (greater than 0). The second parameter, which is optional, is either the value 'start' or 'end', and specifies the point at which the change of values occur within the interval. If the second parameter is omitted, it is given the value 'end'.

cubic-bezier

Specifies a cubic-bezier curve. The four values specify points P₁ and P₂ of the curve as (x1, y1, x2, y2). Both x values must be in the range [0, 1] or the definition is invalid. The y values can exceed this range.

The 'transition-delay' Property

The 'transition-delay' property defines when the transition will start. It allows a transition to begin execution some some period of time from when it is applied. A 'transition-delay' value of '0' means the transition will execute as soon as the property is changed. Otherwise, the value specifies an offset from the moment the property is changed, and the transition will delay execution by that offset.

If the value for 'transition-delay' is a negative time offset then the transition will execute the moment the property is changed, but will appear to have begun execution at the specified offset. That is, the transition will appear to begin part-way through its play cycle. In the case where a transition has implied starting values and a negative 'transition-delay', the starting values are taken from the moment the property is changed.

The 'transition' Shorthand Property

The 'transition' shorthand property combines the four properties described above into a single property.

Note that order is important in this property. The first value that can be parsed as a time is assigned to the transition-duration. The second value that can be parsed as a time is assigned to transition-delay.

An alternative proposal is to accept the font shorthand approach of using a "/" character between the values of the same type. eg. 2s/4s would mean a duration of 2 seconds and a delay of 4 seconds.

Starting of transitions

When the computed value of an animatable property changes, implementations must decide what transitions to start based on the values of the 'transition-property', 'transition-duration', 'transition-timing-function', and 'transition-delay' properties at the time the animatable property would first have its new computed value.

li { transition: background-color linear 1s; background: blue; }
li:hover { transition-duration: 2s; background-color: green; }

Since this specification does not define when computed values change, and thus what changes to computed values are considered simultaneous, authors should be aware that changing any of the transition properties a small amount of time after making a change that might transition can result in behavior that varies between implementations, since the changes might be considered simultaneous in some implementations but not others.

Say something about simulaneity

Once the transition of a property has started, it must continue running based on the original timing function, duration, and delay, even if the 'transition-timing-function', 'transition-duration', or 'transition-delay' property changes before the transition is complete. However, if the 'transition-property' property changes such that the transition would not have started, the transition must stop (and the property must immediately change to its final value).

Implementations must not start a transition when the computed value of a property changes as a result of declarative animation (as opposed to scripted animation).

Implementations also must not start a transition when the computed value changes because it is inherited (directly or indirectly) from another element that is transitioning the same property.

Automatically reversing transitions

A common type of transition effect is when a running transition is interrupted and the property is reset to its original value. An example is a hover effect on an element, where the pointer enters and exits the element before the effect has completed. If the outgoing and incoming transitions are executed using their specified durations and timing functions, the resulting effect can be distractingly asymmetric. Instead, the expected behavior is that the new transition should be the reverse of what has already executed.

If a running transition with duration T, executing so far for duration TE, from state A, to state B, is interrupted by a property change that would start a new transition back to state A, and all the transition attributes are the same (duration, delay and timing function), then the new transition must reverse the effect. The new transition must:

1. Use the B and A states as its "from" and "to" states respectively. It does not use the current value as its from state, due to the rules below.
2. Execute with the same duration T, but starting as if the transition had already begun, without any transition delay, at the moment which would cause the new transition to finish in TE from the moment of interruption. In other words, the new transition will execute as if it started T-TE in the past.
3. Use a timing function that is the portion of the curve traversed up to the moment of interruption, followed in the opposite direction (towards the starting point). This will make the transition appear as if it is playing backwards.
4. Ignore any transition delay.

For example, suppose there is a transition with a duration of two seconds. If this transition is interrupted after 0.5 seconds and the property value assigned to the original value, then the new transition effect will be the reverse of the original, as if it had begun 1.5 seconds in the past.

Note that by using the defined from and to states for the reversing transition, it is also possible that it may reverse again, if interrupted; for example, if the transition reversing to state A was again interrupted by a property change to state B.

Issue: This introduces the concept of reversing a timing function, which the spec has otherwise resisted doing, and also introduces a discontinuity between transitions that have almost completed (which get automatically reversed and thus have their timing function reversed) and transitions that have fully completed (where the reversal doesn't lead to the timing function being reversed). An alternative proposal that avoids this is to follow the normal timing function algorithm, except multiply the duration (and also shorten any negative delay) by the (output) value of the transition timing function of the incomplete transition at the time it was interrupted, and, to account for multiple reverses in sequence, to divide by the shortening applied to the transition being interrupted. For more details see this thread: November 2009 part, December 2009 part, January 2010 part.

Transition Events

The completion of a CSS Transition generates a corresponding DOM Event. An event is fired for each property that undergoes a transition. This allows a content developer to perform actions that synchronize with the completion of a transition.

Each event provides the name of the property the transition is associated with as well as the duration of the transition.

Interface TransitionEvent

The TransitionEvent interface provides specific contextual information associated with transitions.

IDL Definition

  interface TransitionEvent : Event {
    readonly attribute DOMString          propertyName;
    readonly attribute float              elapsedTime;
    void               initTransitionEvent(in DOMString typeArg, 
                                          in boolean canBubbleArg, 
                                          in boolean cancelableArg, 
                                          in DOMString propertyNameArg,
                                          in float elapsedTimeArg);
  };
  

Attributes

propertyName of type DOMString, readonly

The name of the CSS property associated with the transition.

elapsedTime of type float, readonly

The amount of time the transition has been running, in seconds, when this event fired. Note that this value is not affected by the value of transition-delay.

Methods

initTransitionEvent

The initTransitionEvent method is used to initialize the value of a TransitionEvent created through the DocumentEvent interface. This method may only be called before the TransitionEvent has been dispatched via the dispatchEvent method, though it may be called multiple times during that phase if necessary. If called multiple times, the final invocation takes precedence.

Parameters

typeArg of type DOMString

Specifies the event type.

canBubbleArg of type boolean

Specifies whether or not the event can bubble.

cancelableArg of type boolean

Specifies whether or not the event's default action can be prevented. Since a TransitionEvent is purely for notification, there is no default action.

propertyNameArg of type DOMString

Specifies the name of the property associated with the Event

elapsedTimeArg of type float

Specifies the amount of time, in seconds, the transition has been running at the time of initialization.

No Return Value

No Exceptions

There is one type of transition event available.

transitionend

The 'transitionend' event occurs at the completion of the transition. In the case where a transition is removed before completion, such as if the transition-property is removed, then the event will not fire.

• Bubbles: Yes
• Cancelable: Yes
• Context Info: propertyName, elapsedTime

Animation of property types

The following describes how each property type undergoes transition or animation.

• color: interpolated via red, green, blue and alpha components (treating each as a number, see below).
  Issue: Are the colors interpolated in premultiplied space or non-premultiplied space?
• length: interpolated as real numbers.
• percentage: interpolated as real numbers.
• integer: interpolated via discrete steps (whole numbers). The interpolation happens in real number space and is converted to an integer using floor().
• number: interpolated as real (floating point) numbers.
• transform list: see CSS Transforms specification [[!CSS3-2D-TRANSFORMS]].
• rectangle: interpolated via the x, y, width and height components (treating each as a number).
• visibility: interpolated via a discrete step. The interpolation happens in real number space between 0 and 1, where 1 is "visible" and all other values are "hidden".
• shadow: interpolated via the color, x, y and blur components (treating them as color and numbers where appropriate). In the case where there are lists of shadows, the shorter list is padded at the end with shadows whose color is transparent and all lengths (x, y, blur) are 0.
• gradient: interpolated via the positions and colors of each stop. They must have the same type (radial or linear) and same number of stops in order to be animated. Note: [[CSS3-IMAGES]] may extend this definition.
• paint server (SVG): interpolation is only supported between: gradient to gradient and color to color. They then work as above.
• space-separated list of above: If the lists have the same number of items, each item in the list is interpolated using the rules above. Otherwise, no interpolation (unless stated otherwise above). For lists that represent a list of things to be cycled through (e.g., stroke-dasharray), it is straightforward to interpolate between lists of different lengths by repeating the two lists out to the least common multiple of their lengths.
• a shorthand property: If any part of a shorthand can be animated, then interpolation is performed as if those animatable properties were individually specified.

Issue: Need to describe handling of out-of-range values that can result from cubic-bezier(). Clamping values to the allowed range is probably the best solution.

Animatable properties

Properties from CSS

This list omits the following properties that Gecko can animate, and which likely should be included: background-size, border-*-radius, box-shadow, column-count, column-gap, column-rule-color, column-rule-width, column-width, font-size-adjust, font-stretch, marker-offset, text-decoration-color, transform.

Properties from SVG

All properties defined as animatable in the SVG specification, provided they are one of the property types listed above.

References

Normative references

Other references

Property index

Index